Controlling corrosion in Saudi Arabia’s desalination plants

Posted by Joanne McIntyre

In Saudi Arabia desalination fulfills the most basic need for humanity, providing clean, safe water. The many plants constructed along the eastern and western coasts literally pump life into this arid country. Keeping these plants running and tackling the inevitable corrosion issues that arise from processing seawater is the responsibility of the corrosion department within the Desalination Technologies Research Institute (DTRI).

Stainless Steel World interviewed Dr. Abdelkader Meroufel, Head of the Corrosion Department, about the desalination technologies employed in Saudi Arabia and some of the challenges this involves.___

As Head of Corrosion Department

at the Desalination Technologies

Research Institute Dr. Meroufel’s main

responsibility is to provide technical

corrosion consultancy support

to operating and maintenance of

desalination plants. Interestingly, he

outlined the absence of professional

corrosion engineering practices such

as in the oil & gas industry. Corrosion

problems are managed by mechanical

engineers and chemists who have

limited knowledge of corrosion

mechanisms.

He elaborates: “Desalination is not

like the oil & gas or petrochemical

industries which have traditionally

undertaken a great deal of corrosion

engineering activity. In desalination,

generally the end users follow the

recommendations of the manufacturer

in terms of materials, inspection and

maintenance. If problems emerge

during operation, the operator then

needs to look for experts to provide

a solution. Currently no corrosion

engineering programs are offered by

local universities in the Middle East

so there is a lack of knowledge in

this area; we have no local corrosion

engineering graduates.

This is

generally overcome by regional NACE

sections which provide specialized

corrosion-related courses for short

duration.”

Another important task which falls

under Dr. Meroufel’s responsibility is

conducting applied research projects

related to corrosion in different areas of

the desalination industry.

“The role of DTRI is to provide

technical support to desalination

plants on a wide range of issues,”

explains Dr. Meroufel. “Our activity

is split into various fields such as chemistry, marine biology & environment, corrosion, thermal process engineering, reverse osmosis engineering, etc. As a corrosion department, we investigate plant failures and provide recommendations, and test new materials and chemicals that could affect the integrity of equipment in desalination plants.

Our activity is mainly focused on applied research, studying existing technology and finding its limitations, how we can use and adapt it to the needs of our customers. It’s a very practical, application-focused approach. However, we also carry out

some fundamental research through collaborations with universities or companies which are working on the development of new solutions for the desalination sector.”

Desalination technologies

The main desalination technologies used in the Middle East are thermal and membrane. Each method differs in terms of working principle, energy consumption, cost and reliability for seawater treatment. The most common desalination technologies in the region are:

Multistage flash (MSF) – thermal distillation

Multi-effect distillation (MED) – thermal distillation

Reverse osmosis (RO) – membrane

“The various thermal and membrane desalination processes each present their own set of corrosion, materials and environmental challenges, requiring wide-ranging areas of research,” continues Dr. Meroufel. “These include corrosion occurring in hot vapour environments, aerated and nonaerated seawater, potable water, soil for water transmission pipelines, as well as in storage tanks and boilers. Troubleshooting investigations and failure analysis are important tasks for our team. Any equipment that suffers corrosion-related failures or environmentally assisted cracking is investigated to determine the root causes and provide recommendations to avoid repetition.”

Apart from his ongoing research, Dr. Meroufel is currently involved in two large-scale projects. “We are involved in a worldwide project managed by the French Corrosion Institute to investigate crevice corrosion of stainless steels and nickel-based alloys. The objective is to examine various grades of stainless steels that have been exposed to natural seawater at locations around the world. This will provide very valuable information. For example, we will see how biofilm – which is different in the North Sea compared to the Gulf Sea – impacts on the initiation and propagation of crevice corrosion.

We are also looking at the impact of seawater temperatures which vary greatly around the planet. Crevice geometry is another factor we are studying. Initial results were presented during the European Desalination Society conference in 2016 and further results will be presented soon.” “This project has an impact on reverse osmosis membrane desalination in particular. Since we use a large amount of corrosion-resistant alloys (CRA) we are very satisfied with this project as the result will provide us with a clear picture when combined with information from the field and feedback from our desalination plants. We are now trying to make a correlation between the findings of the study and the experiences of our colleagues at the desalination plants.”

The second large project Dr. Meroufel is involved in relates to once-through MSF thermal desalination operating at Top Brine Temperature (TBT) of 130°C, which could compete with MSF brine recycle. “Once-through MSF is an ‘old’ technology which fell out of favour for two main reasons: the high cost of chemicals required to treat the seawater and the lack of advanced materials available at the time. We are now reevaluating this process using different grades of stainless steel, titanium and cupronickel alloys, working at high temperatures of up to 130°C. The target is to achieve a high performance ratio compared to brine recycle MSF. This would deliver savings in terms of both energy consumption and the construction footprint of the plant. We are working in collaboration with a Doosan R&D partner; preliminary results will be presented during the 2017 International Desalination Association (IDA) Congress in São Paulo and the 2017 Stainless Steel World Conference in Maastricht.”

Localized corrosion & biofouling

Aged thermal desalination plants face more corrosion issues than the newer RO technology, Dr. Meroufel explains: “Existing plants in Saudi Arabia were mostly constructed in the late 1970s/ early 1980s. At that time the industry wasn’t aware of some problems that could occur during operation. It’s difficult to modify the design during operation, so the reality is that we need to find ways to treat the seawater going into the unit to avoid corrosion. One of the main problems is the continuous operation of the plants. Water demand is growing rapidly with the population and this puts enormous pressure on the plants, making it difficult to respect the interval between periodic inspections that are needed. We are forced to wait for longer intervals and this creates specific problems. One major challenge for the CRAs we use in the high temperatures and more severe conditions in plants today is localized corrosion. There is no way to monitor or detect this corrosion unless we stop the unit and open it up for inspection. Monitoring localized corrosion is a major challenge.”

“Another significant challenge is the problem of biofouling, a normal phenomenon that cannot be avoided even with chemical treatment. It plays a specific role especially within our RO desalination plants. In thermal MSF and MED plants biofouling doesn’t cause a lot of problems, but in RO plants it creates problems both for the membranes and for metallic materials like super austenitics, duplex and super duplex.”

As desalination technology has matured the materials specified have also been modified. “In the early days, seawater desalination plants utilized a lot of Ni-Resist alloy, an austenitic cast iron in seawater or brine handling pumps. However, in response to the problem of stress corrosion cracking, we recommended to plant maintenance engineers that this material be replaced with duplex 2205. Based on feedback, there have been no problems using this material. Today we largely use a combination of two materials: stainless steel 316L and duplex 2205. In RO plants, super austenitics are the most commonly used materials, especially for high-pressure piping and pumps.

In some cases plants are also using super duplex 2750.” An innovation at the Ras al Khair MSF plant on the eastern coast, which was commissioned in 2015, was the use of duplex 2205 as the evaporator shell material. “Previously the materials specified were carbon steel for the low-temperature stages and carbon steel clad with 316L for the hightemperature stages. However for this new construction it was decided to use solid duplex for the entire evaporator shell.” “Materials used for tubing inside the heat exchangers are also evolving. In situations where the operating conditions are very aggressive titanium grade 2 is specified, otherwise copper nickel 90/10 or modified 70/30 are the two main materials most often specified.”

“Most of the corrosion solutions we propose are related to CRAs when our studies show that new materials in the market could be suitable solutions depending on the situation. Another emerging possibility, however, is the shift from metallic material to composites, particularly for heat exchangers. The implementation of reinforced polymers in this application is a solution that we are exploring for the future. Composites can be a viable option in terms of cost, corrosion performance and degradation. The degradation characteristics of composite materials need further study and comparison to the degradation of metallic materials. However we already know that reinforced polymer composites can offer a good compromise between mechanical and heat transfer properties.”

Dr. Abdelkader Meroufel graduated in 2006 from La Rochelle University in France with a PhD in materials science. His research activity started with active materials such as carbon and low-alloy steels, focusing on zinc-rich powder coating including conducting polymers; and CO2/H2S corrosion in thin layer for flexible pipeline design. He has been assistant professor at La Rochelle and Jazan (Saudi Arabia) universities. In between he gained experience as an associate researcher at the French Petroleum institute. Dr. Meroufel joined the DTRI in 2012 as a corrosion specialist and has headed the corrosion department since 2014. He also conducts failure analysis related to corrosion or environmental assisted cracking based failures.